Various types of magnetic tapes are presently in use for magnetic-tape sound recording. Magnetic-tape sound recorders are, thus, usually designed to operate with a proper frequency response characteristic depending upon the types of the tapes to be used on the recorders. Since, however, the frequency response characteristic of a magnetic-tape sound recorder varies from one tape to another even when the tapes may be of the same type or make, it is desired to adjust the frequency response characteristic of the recorder for each of the tapes to be used on the recorder to achieve sophisticated input-output characteristics in the recorder. For this purpose, it has been proposed and put into practice to provide a magnetic-tape sound recorder with a recording signal level meter adapted to indicate the signal level detected in the recording line of the recorder for enabling the operator of the recorder to visually determine the detected level of the recording signal. The level meter provided in an existing magnetic-tape sound recorder is used in combination with manually-operated signal level control means for adjusting the recording signal level of the recorder in such a manner that signals of a preselected level are supplied to the recording head of the recorder when the level meter registers the signal level of zero volume unit (vu) in response to input signals of a relatively low, predetermined frequency of for example 400 hertz. In another type of magnetic-tape sound recorder having a recording signal level meter, the level meter is used in combination with manually-operated frequency response compensating means for adjusting the amount of compensation for the frequency response of the recorder on the basis of an input signal of a relatively high predetermined frequency of for example 10 kilo hertz. The adjustments of the recording signal level and the amount of compensation for the frequency response characteristic in these known magnetic-tape sound recorders are, thus, manually conducted by having recourse to reading the level meters. Not only elaborate and skilled techniques but time-consuming efforts are therefore indispensable for the adjustment of the frequency response characteristics of known magnetic-tape sound recorders. The present invention contemplates provision of a simple and economical device which will enable the operator of a magnetic-tape sound recorder to easily and accurately adjust the frequency response characteristic of the recorder depending upon the individual types and makes of the magnetic tapes to be used on the recorder.